1. Field of the Invention
This invention relates generally to sizing compositions, methods and apparatus for cleaning sized fiber glass strands, yarns and fabrics without the use of high temperature thermal processes.
2. Technical Considerations
Glass fiber strands are coated during the forming process with a sizing composition to protect their surfaces from abrasion and scratching. Generally, not only are the sizing compositions formulated to protect the fibers during forming, but also to provide the fiber strands with requisite characteristics and properties needed for further processing. In the case of fiber strands used to form yarns to be woven into fabrics, in addition to protecting the fibers during forming, the sizing composition must also provide for good weavibilitity, particularly on air-jet looms. Typically, such fibers are combined with polymeric matrix materials to form composites or laminates so it would also be advantageous if the sizing composition was compatible (i.e. provided for good wetting and bonding between the matrix material and the surface of the glass) with the polymeric matrix material. However, typical sizing compositions used on glass fiber strands formed into fabrics or mats to be incorporated into printed circuit boards are not compatible with the polymeric matrix material and must be removed prior to incorporation.
Typical sizing compositions used for textile (i.e. fabric) applications are disclosed in K. Loewenstein, The Manufacturing Technology of Continuous Glass Fibers (3rd Ed., 1993) at pages 238-241 which are hereby incorporated by reference. Such sizing compositions generally comprise modified starches (such as partially or fully dextrised starches) and oils (such as hydrogenated vegetable oils) that are not compatible with the conventional resin matrix materials used to form laminates for printed circuit board applications. While it is possible to incorporate fibers having starch/oil sizings on their surfaces into such resin matrix materials to form composites, the mechanical properties (particularly flexural strength) and hydrolytic stability of the composites are typically not acceptable for use in printed circuit board applications. Therefore, prior to incorporating fabrics made from fibers coated with such sizing compositions into the resin matrix, the sizing composition must first be removed (or de-oiled) from the surface of the fibers. The sizing removal is typically accomplished by a heat cleaning or washing operation.
Heat cleaning is a process that is both time consuming and deleterious to the mechanical properties of the glass. Conventional heat cleaning processes involve heating the glass fabric at about 380xc2x0 C. for 60-80 hours. After cleaning, the fabric must be re-coated with a coating or finishing size that is compatible with the polymeric matrix material. The finishing size typically includes a silane coupling agent that promotes wetting of the polymeric matrix material on the surface of the glass fiber as well as coupling (or bonding) between the glass fiber and the polymer. However, heat cleaning processes are not always completely successful in removing the non resin-compatible materials from the glass surface and can further provide for contamination of the fabric with the products of decomposition from the sizing material. Additionally, it has been observed that the tensile strength of glass fibers exposed to elevated temperatures decreases with increasing temperature. See P. K. Gupta, xe2x80x9cGlass Fibers for Composite Materials,xe2x80x9d Fiber Reinforcements for Composite Materials. Composite Materials Series, 2 , A. R. Bunsell, ed. (1988) at pages 19-71 and W. H. Otto, Proceedings of Sixth Sagamore Ordinance Materials Research Conference on Composite Materials and Composite Structures, (1959), which are hereby incorporated by reference. While the application of a finishing size to the glass fiber fabric can increase the tensile strength of the fabric after heat cleaning, the tensile strength of the finished fabric is still lower than that of fabric that has not been heat cleaned.
Further disadvantages of the heat cleaning process are the large floor space required for the heat cleaning equipment, as well as the cost and maintenance of the heat cleaning equipment itself.
One alternative to de-oiling by heat cleaning is to wash the fabric with a solvent, preferably water, to remove the sizing composition. However, water washing can require the re-formulation of the sizing composition to provide for increase water solubility. For example, Japanese Patent Application No. 8-119-682 discloses a primary sizing composition containing a water-soluble epoxy resin that can be easily removed by rinsing with water. Non-aqueous solvent cleaning is generally not desirable for environmental and safety reasons.
It would be advantageous to provide a method and apparatus for de-oiling a variety of sizing compositions without the use of high temperature thermal processes and without the need for substantial re-formulation of conventional sizing compositions, that is environmentally friendly and can be done in-line with a other processing equipment. Additionally, there is a need for sizing compositions that can be easily de-oiled without the use of high temperature thermal processes.
One aspect of the present invention is a sizing composition for use in a fiber forming operation, the sizing composition comprising: a photocatalytically degradable organic material; and a photocatalyst material, wherein the photocatalyst material is capable of degrading at least a portion of the photocatalytically degradable organic material upon exposure to at least one radiation source having a wavelength sufficient to activate the photocatalyst material.
Another aspect of the present invention is a fiber strand comprising a plurality of individual filaments, at least one filament of the plurality of individual filaments comprising a residue of a coating composition positioned upon at least a portion of a surface of the at least one filament, the coating composition comprising: a photocatalytically degradable organic material; and a photocatalyst material, wherein the photocatalyst material is capable of degrading at least a portion of the photocatalytically degradable organic material upon exposure to at least one radiation source having a wavelength sufficient to activate the photocatalyst material.
Still another aspect of the present invention is a fabric comprising one or more fiber strands comprising a residue of a coating composition positioned upon at least a portion of a surface of at least one of the one or more fiber strands, the coating composition comprising: a photocatalytically degradable organic material; and a photocatalyst material, wherein the photocatalyst material is capable of degrading at least a portion of the photocatalytically degradable organic material upon exposure to at least one radiation source having a wavelength sufficient to activate the photocatalyst material.
Another aspect of the present invention is a method of forming a fiber strand, the method comprising the steps of: attenuating a plurality of individual filaments from a fiber forming apparatus; applying a coating composition to at least a portion of the surface of one or more of the plurality of individual filaments, the coating composition comprising a photocatalytically degradable organic material, and a photocatalyst material; gathering the plurality of individual filaments together to form a coated fiber strand; and exposing the coated fiber strand to at least one radiation source having an wavelength sufficient to activate the photocatalyst to photocatalytically degrade at least a portion of the photocatalytically degradable organic material.